Conventionally, in a radio communication system to which long term evolution (LTE) is applied, a communication controlling procedure referred to as closed-loop precoding is used in down link (DL) from a base station (eNB: eNodeB) to a mobile station (UE: User Equipment). In the closed-loop precoding, the base station uses spatial multiplexing to simultaneously transmit a plurality of data streams and rank adaptation to adaptively control the number of data streams (spatial layers) (transmission ranks) to be spatially multiplexed together to form a directional beam by multi antenna. The mobile station selects an optimal one from a precoding matrix defined for each rank to feed back to the base station.
Specifically, in the closed-loop precoding, the mobile station feeds back channel state information (CSI) indicating a channel state to the base station. The above-described CSI includes a rank indicator (RI) indicating a recommended transmission rank, a precoding matrix indicator (PMI) indicating a recommended precoding matrix, and a channel quality indicator (CQI) indicating radio channel quality when the above-described RI and PMI are assumed. When the base station receives the above-described CSI from the mobile station, this applies the precoding matrix determined based on the above-described RI and PMI to UE-specific reference signals (RS) and a physical downlink shared channel (PDSCH) to transmit to the mobile station. The mobile station which receives the above-described precoding matrix demodulates the PDSCH by using a channel estimation value based on the UE-specific RS.
As described above, a conventional CSI feedback method assumes PDSCH transmission to which single user-multiple input multiple output (SU-MIMO) being spatial multiplexing technology of signals to one mobile station is applied. Therefore, the base station selects a rank according to the radio channel quality and the precoding matrix for the rank putting importance on transmission efficiency to one mobile station on the assumption that reliability is assured by retransmission control.
Non Patent Document 1: IEEE 802.16 Broadband Wireless Access Working Group Closed Loop MIMO Precoding (Nov. 4, 2004), http://www.ieee802.org/16/tge/contrib/C80216e-04_293r2.pdf
However, technology of dynamically switching between the above-described SU-MIMO and MU-MIMO (multiple user-multiple input multiple output) being spatial multiplexing technology of signals to a plurality of mobile stations is recently developed. Technology of applying the closed-loop precoding to an EPDCCH (enhanced physical downlink control channel) supporting the spatial multiplexing in addition to conventional time multiplexing is also developed. Therefore, expansion of the CSI feedback method from the mobile station to the base station is desired in order to keep line with the technology.
A feedback method of a multiple CSI process is suggested, for example, as a new CSI feedback method. In such method, the mobile station has codebook subset restriction for each CSI process and feeds back the CSI within a range of the RI and PMI restricted according to a bitmap indicated by a higher layer. As an application example, the mobile station feeds back the CSI assuming the SU-MIMO of the recommended rank of a connected cell in a CSI process 1. The mobile station feeds back the CSI assuming the MU-MIMO and the EPDCCH of a rank 1 of the connected cell in a CSI process 2. Furthermore, the mobile station feeds back the CSI assuming the MU-MIMO and the EPDCCH of a rank 2 of the connected cell in a CSI process 3.
In this manner, in the above-described CSI feedback method, the mobile station transmits the CSI (for example, 12 bits) two or three times as large as conventional one to the base station though system performance is improved. Overhead of control information transmitted by the mobile station to the base station at the time of the CSI feedback significantly increases with this.